In recent molecular classifications the family Woodsiaceae has been radically pruned and has lost almost all the genera with which has been associated over recent decades. Familiar genera such as Athyrium, Deparia and Diplazium are now placed in the Athyriaceae. Cystopteris and Gymnocarpium are now included in the Cystopteridaceae and the Onocleaceae includes Matteuccia and Onoclea. Woodsiaceae is now restricted to Woodsia and a handful of closely related segregate genera.
Until a few weeks ago no comprehensive molecular study of the genus had been completed but now 2 unrelated papers have been published. What is more, although the 2 studies have reached similar conclusions their final taxonomies differ substantially.
In the first to be published in June by A. Shmakov, The New System of Woodsiaceae in Turczaninowia (18)2, pp. 11-16 the author concluded that Woodsia should be split into 7 genera. These are (in alphabetical order) Cheilanthopsis, Eriosoriopsis, Hymenocystis, Physematium, Protowoodsia, Woodsia and Woodsiopsis. Under this classification Woodsia alpina, W. ilvensis and W. polystichoides are retained in Woodsia but most of the North American species are included in Woodsiopsis. Woodsia fragilis is the sole member of Hymenocystis.
The second study was published earlier this month by Y. Shao et al. Molecular Phylogeny of the Cliff Ferns (Woodsiaceae: Polypodiales) with a Proposed Infrageneric Classification in the open access journal PLoS ONE 10(9). Although their findings are similar they have concluded that the most practical taxonomic arrangement is to include all the species in Woodsia that is then subdivided into 3 subgenera.
I suspect that the latter classification will be adopted by most pteridologists although further research may prove me wrong.
While researching this post I have noticed that Woodsia fragilis that is occasionally seen in cultivation has an invalid name as it is a later homonym. The correct name is Woodsia caucasica (or Hymenocystis fragilis if you prefer to split).
Here is a magnificent example seen recently in Bridget Laue’s garden.